There are bonus pokemon you can send to your Diamond/Pearl for free. These pokemon are usuallly quite strong. Below are these pokemon:

Pikachu

When you defeat Pokétopia Master Mysterial, you will be able to download a Pikachu to your Diamond/Pearl in the shop. It can be downloaded to multiple games, and won't cost a cent. The Pikachu is a level 10 female holding a Light Ball. It knows Volt Tackle, Surf, Tail Whip, & Thunder Wave.

Magmortar

You can download Magmortar with the code B416-X4HT-VTWF. Magmortar is a level 50 female holding a Charcoal. It knows Psychic, Flamethrower, Hyper Beam, & Solar Beam. It is an Adament nature.

Electivire

You can download Electivire with the code BA16-X4SH-E2AT. Electivire is a level 50 male holding a Magnet. It knows Thunder Punch, Ice Punch, Cross Chop, & Earthquake. It is a Modest nature.